1. Field of the Invention
The present invention relates to heterojunction bipolar transistors having a double heterostructure and more particularly, to a heterojunction bipolar transistor having an improved base and collector structure and an improved base and emitter structure.
2. Description of the Related Art
A heterojunction bipolar transistor (hereinafter referred to as HBT) having an emitter region made of semiconductor material that is larger in band gap than a base region, has been widely studied as a microwave transistor or a high-speed logical circuit transistor, since the heterojunction transistor is more excellent in high frequency and switching characteristics than a homojunction bipolar transistor.
The HBT using GaAs or AlGaAs as the semiconductor material, in particular, has been increasingly considered to offer a very promising prospect as a superhigh speed device, since the transistor is larger in carrier mobility than a transistor using Si. However, because such compound semiconductor as GaAs or AlGaAs is more expensive and brittler than Si, the HBT using such material has been considered to be rather unsuitable in the current circumstances for its mass production when compared with the transistor using Si.
In order to overcome such circumstances, there have been made attempts to introducing the heterojunction into the Si bipolar transistor to attain its higher performances. And there has been recently reported a so-called silicon-based HBT having a wide gap emitter of Si.sub.1-x C.sub.x (0.0&lt;x.ltoreq.1.0) grown on Si or having a narrow gap base of Si.sub.1-x Ge.sub.x (0.0&lt;x.ltoreq.1.0, hereinafter x is assumed to be in the same range) grown on Si, which has been made on an experimental basis. In particular, the latter Si.sub.1-x Ge.sub.x /Si HBT, in which a base layer is made of material smaller in band gap than Si and thus its turn-on voltage is small, requires less power consumption than the prior art Si bipolar transistor. In addition, since Si.sub.1-x Ge.sub.x /Si HBT uses a base layer made of mixed crystal Si.sub.1-x Ge.sub.x, a so-called graded base structure wherein an electric field for acceleration of carriers is provided in the base can be employed therefor and thus the transistor can be operated at a speed faster than that of the prior art Si bipolar transistor.
In this way, the Si.sub.1-x Ge.sub.x /Si HBT, when compared with the prior art homojunction silicon bipolar transistor, requires less power consumption and produces a higher operational speed, but has a problem which follows, because of its so-called double heterostructure, i.e., having the other heterojunction also in the collector side.
FIG. 18 shows a diagrammatical cross-sectional view of an epitaxial wafer used in a prior art Si.sub.1-x Ge.sub.x /Si series HBT. The epitaxial wafer is obtained usually by a molecular beam epitaxy (MBE) technique, a chemical vapor deposition (CVD) technique based on a limited reaction processing (LRP), or the like technique.
The HBT is prepared with use of such a wafer. That is, the wafer is made by sequentially epitaxially growing an n.sup.- type Si layer 21 of 200 nm thickness and having a doping concentration of 1.times.10.sup.20 cm.sup.-3 as a collector contact layer, an n.sup.- type Si layer 22 of 500 nm thickness and having a doping concentration of 5.times.10.sup.16 cm.sup.-3 as a collector layer, a p.sup.+ type Si.sub.0.9 Ge.sub.0.1 layer 23 of 50 nm thickness and having a doping concentration of 1.times.10.sup.19 cm.sup.-3 as a base layer, an n type Si layer 24 of 150 nm thickness and having a doping concentration of 1.times.10.sup.17 cm.sup.-3 as an emitter layer, and an n.sup.+ type Si layer 25 of 100 nm thickness and having a doping concentration of 1.times.10.sup.20 cm.sup.-3 as an emitter cap layer all formed on a p.sup. - type Si substrate 20. The impurity doping concentrations for the respective epitaxial layers are set to become uniform. The thicknesses of the respective layers and dopant types illustrated in FIG. 18 refer to so-called design values set prior to the execution of the epitaxial growth.
Shown in FIG. 19 an energy band profile within a device when it is assumed that the dopants are activated as carriers in a condition where the thicknesses of the respective layers of a wafer and the impurity distributions are the same as design values. In the drawing, electron energy is expressed as positive. As will be seen from the drawing, an energy band discontinuity at a heterojunction interface between the Si.sub.0.9 Ge.sub.0.1 and Si layers is about 0.1 eV, 80% of which, i.e., 0.08 eV appears as a band discontinuity on the side of a valence band and the remainder, i.e., 0.02 eV, that is very small, appears as a band discontinuity on the side of a conduction band. It is known that such tendency is not changed and the band discontinuity value of the conduction band side is not changed and substantially constant even when a content of Ge to Si is increased to a fairly high level. Accordingly, in the case of an npn type bipolar transistor, the band discontinuity of the conduction band side is very small even when the heterojunction interface between the Si.sub.0.9 Ge.sub.0.1 and Si layers is abruptly changed, so that electrons can smoothly flow from an emitter to a collector while holes can be effectively trapped within a base layer by means of a potential barrier caused by the large band discontinuity of the valence band side. Thus, it will be appreciated that, so long as at least this material series is employed, the use of the abrupter heterojunction enables the transistor to have a higher performance.
In the course of actually fabricating a transistor with use of the aforementioned epitaxial wafer, the wafer experiences various thermal histories including epitaxial growth itself. That is, heating the wafer to a high temperature causes a doping profile so far set to be inevitably changed due to thermal diffusion. In particular, the base layer is as thin as at most 100 nm and very high in doping concentration, the thermal diffusion of the dopant within the base layer becomes a problem.
FIG. 20 shows an impurity profile in a wafer of a resultant transistor completed after the wafer of FIG. 18 experiences various thermal histories, in which a design impurity profile is represented by solid lines and the final impurity profile is by broken lines and two-dot chain lines. It will be seen from FIG. 20 that, since the dopant of the base layer diffuses into the emitter and collector sides and p type and n type dopants are effectively changed as compensated for, the heterojunction and the p-n junction are shifted from their set positions.
Shown in FIG. 21 is a model form of energy band profile corresponding to the effective doping profile of the final transistor shown in FIG. 20. In FIG. 21, electron energy is set to be positive as in FIG. 19. It will be seen from the drawing that, since the collector is usually set to be lower in impurity concentration than the emitter for the purpose of making small the capacitance of the base/collector junction, the effective impurity type in the vicinity of the base/collector junction in the collector is p type due to the diffusion of the p type dopant from the base layer thereto, which results in that the heterojunction is formed in the p type region and a potential barrier to electrons is formed in the conduction band.
For this rason, so long as such an epitaxial wafer film structure as in the prior art is employed, not only it is impossible to obtain a previously set doping profile for the completed final transistor but also the potential barrier formed on the conduction band side prevents the flow of electrons from the base to the collector. This results in that the electron transportation efficiency is remarkably reduced and thus the current amplification factor and the operational speed are reduced, which becomes a serious problem in the transistor fabrication. This problem is not limited to the aforementioned Si.sub.1-x Ge.sub.x /Si HBT and holds true even for an HBT of an npn type and of a double heterostructure using such semiconductor material having the band discontinuity of the heterojunction appearing mainly on the valence band side as InP/GaInAsP compound semiconductor material and when employing such an epitaxial wafer film structure as in the prior art. Further, even when an HBT of a pnp type and of a double heterostructure is to be prepared using such materials having the band discontinuity appearing mainly on the conduction band side as AlGaAs and GaAs, this problem becomes serious.
In this way, the HBT of the double heterostructure using such semiconductor material that the band discontinuity of the heterojunction appears mainly on the valence band side has had a problem that, since the heterojunction and the p-n junction, in particular, at the base/collector junction are shifted by the thermal histories, it is difficult to realize a high current gain and a high speed operation inherent in an HBT.
In an HBT, it is common practice to make thin a base region for the purpose of obtaining a high speed operation. However, making thin the base region, at the same time, causes a base sheet resistance to be increased. To avoid this, it is desirable to set the base region at a high impurity concentration.
Meanwhile, it has been so far considered as taught, for example, in a paper entitled "Heterostructure Bipolar Transistor and Integrated Circuits", H. Kroemer, in the Proc. IEEE, Vol. 70, No. 1, pp. 13-25, January 1982, that an emitter region is set preferably at a low impurity concentration because a capacitance C.sub.E of an emitter/base junction is set small in order to shorten an emitter charging time. However, it has been found through simulation that the setting of the emitter region at a high impurity concentration enables the operation of the transistor up to its high current density zone to realize a high speed operation (for example, refer to a paper entitled "Characteristic Analysis of Si.sub.1-x Ge.sub.x /Si-HBT based on DC one-dimensional Model", Endo, et al., in the Proceeding of the Fiftieth Meeting of The Japan Society of Applied Physics, 28p-A-16, 1989). According to this paper, as the impurity concentration of the emitter increases, transconductance g.sub.m increases beyond the corresponding amount of capacitance increase of the emitter/base junction and thus the emitter charging time decreases. That is, we can consider that an HBT having a high concentration emitter can be operated faster than an HBT having a low concentration.
As has been explained above, since the base impurity concentration of the HBT is high by nature, the increased emitter impurity concentration causes the thickness of the depletion layer region at the heterojunction interface to be decreased, so that an electric field created at the p-n junction becomes extremely strong and thus the voltage withstanding characteristic becomes deteriorated. This is unfavorable from the viewpoint of the actual device applications. For the purpose of overcoming it, there has been proposed an HBT structure wherein a base region is made up of two base layers, a first one of which is provided on the side of an emitter and is set to have such a thickness W.sub.B1 and a low impurity concentration N.sub.B1 that satisfy the following relationship (1), whereby the base region becomes a complete depletion layer region in a thermally balanced state and thus the high concentration emitter can be realized without causing deterioration of a voltage withstanding characteristic (refer to Japanese Patent Disclosure (Kokai) No. 59-211266). EQU N.sub.B1 W.sub.B1.sup.2 .ltoreq.2.epsilon..sub.B .epsilon..sub.O V.sub.bi /q(1)
where, q is the unit quantity of electric charge (1.60.times.10.sup.-19 C.), .epsilon..sub.O is dielectric constant in vacuum (8.86.times.10.sup.-14 F./cm), .epsilon..sub.B is the relative dielectric constant of the first base layer, and V.sub.bi is a built-in potential for a p-n junction made up of the emitter region and the first base layer.
Meanwhile, it is known that, when the base region is made of compound semiconductor material such as Si.sub.1-x Ge.sub.x that allows change of an energy band gap by changing a composition ratio x of the compound semiconductor, the employment of a graded base structure is high effective for realizing a transistor having a high operational speed. To this end, in the transistor of the prior art structure, grading is provided throughout the entire base region to maximize the aforementioned effect.
Now consider the aforementioned 2-layer base structure is employed for the prior art graded base structure subjected to the grading therethroughout. In this case, the grading starts from an end of the emitter side of the first base layer having a low impurity concentration. However, since the first base layer becomes a complete depletion layer in a thermally balanced state under such a condition as to satisfy the relationship (1), the grading effect at that part disappears, which is unpreferable from the viewpoint of realizing a high speed transistor.
Meanwhile, in the case of the Si.sub.1-x Ge.sub.x /Si HBT wherein the collector (or emitter) region is different from the base region in the lattice constant of the semiconductor material, it is impossible to set the thickness of the base region at a value larger than its critical film thickness. The term "critical film thickness" as used herein means the maximum thickness of the thin film up to which material different in lattice constant from the substrate can be grown on a substrate by an existing crystal growth technique without causing any lattice defects or shift at the heterojunction interface. When grading is applied throughout the entire base region as in the aforementioned base of the prior structure, since the first base layer of the base region in which no grading effect appears and the second base layer in which the grading effect appears are both made of the same semiconductor material, the total thickness of the base region becomes larger by an amount corresponding to the provision of the first base layer and thus the base is subject more largely to the restriction of the aforementioned critical film thickness.
That is, there has existed a problem that the feature of the graded base structure cannot be effectively exhibited by means of a mere combination of the prior art 2-layer base structure and the graded base structure and the combination structure tends to be easily limited by the crystal growth technique.
In this way, it has been difficult to effectively exhibit the feature of the graded base structure and for the crystal growth technique to less limit the combination structure and to realize an HBT having excellent voltage withstanding and high-speed characteristics, by means of the mere combination of the prior art 2-layer base structure and the graded base structure.
Further, when the first and second base layers are made of the same semiconductor material, since the thickness of the base region becomes larger by an amount corresponding to the provision of the first base layer, the 2-layer base structure will be more strongly subject to the aforementioned the limitation critical film thickness.
The limitation can be lightened by making small a difference in lattice constant between the semiconductor materials of the collector or emitter region and the base region. However, the changing of the composition ratio of the base material to make small the difference in lattice constant between the base and collector or emitter has involved such a problem that, since this is usually equivalent to making small a difference in energy band gap between the both, the feature of the heterojunction cannot be sufficiently exhibited.
In this way, in the case of the HBT of the prior art 2-layer structure of the emitter and base, when the semiconductor material of the emitter or collector is different in lattice constant from that of part of the base region, the structure is limited by the critical film thickness and thus it is difficult to realize an excellent HBT having a high speed operational performance.
In view of the above circumstances, it is an object of the present invention to provide a layer structure in an epitaxial wafer for use in fabricating an HBT of an npn type which has a base/collector junction of an abrupt heterojunction and uses a semiconductor material causing band discontinuity of the heterojunction to appear mainly on the side of a valence band, which layer structure can prevent formation of a potential barrier to electrons in the vicinity of a base/collector junction of a conduction band to thereby obtain a smooth conduction band profile allowing quick flow of electrons.
Another object of the present invention is to provide a layer structure in an epitaxial wafer for use in fabricating an HBT of a pnp type which has a base/collector junction of an abrupt heterojunction and uses a semiconductor material causing band discontinuity of the heterojunction to appear mainly on the side of a conduction band, which layer structure can prevent formation of a potential barrier to positive holes in the vicinity of a base/collector junction of conduction band to thereby obtain a smooth valence band profile allowing quick flow of positive holes.
A further object of the present invention is to realize an HBT having excellent voltage withstanding and high operational speed characteristics, which can sufficiently exhibit the feature of a graded base structure and can lighten the restrictions of a crystal growth technique.
Yet a further object of the invention is to realize an HBT of a 2-layer structure of an emitter and a base having a high operational speed characteristic, which can lighten any restriction of a critical film thickness caused by the limitations of a crystal growth technique.